Abstract: Effects of mercuric chloride (MC) on the reproductive performance of two successive generations of rats were evaluated in our laboratory. Significant differences resulting from exposure of the F0 generations to MC were found in implantation efficiency, fertility, live births, day 4 survival indices, litter size, and body weights of F1 pups. Limitations of this study design were its inability to determine which sex, male, female or both, may have contributed most to the reproductive and fertility effects, and the lack of a detailed semen evaluation that is critical in the assessment of the male reproductive system after exposure to environmental toxicants. In order to determine which sex was responsible for adverse effects on reproduction, a crossover mating trial is planned with the control and high dose groups. The long-term goals of the research proposed here are to 1. Test the hypothesis that reduced fertility seen in the F0 generation of the study by Atkinson et al 2001 results from exposure of male rats to MC. 2. Test the hypothesis that MC-induced reduced fertility may have resulted from deficits in epididymal sperm function (number, morphology and motility) and/or lower testosterone. 3. Test the hypothesis that MC-induced decreased testicular weights and serum testosterone levels may have resulted from increased apoptosis in germ and/or somatic cells.